The dhcprelay agent doesn't require a configuration file as the DHCP server does; you just need to launch a binary on the command line. What you must know is the IP address of the DHCP server that's located elsewhere on the network that your gateway is connected to. Once you've launched dhcprelay , it forwards requests and responses between the client on your wireless network and the DHCP server located elsewhere on the ISP or corporate network:
dhcprelay -i ral0 10.42.42.42
In this case, it relays requests from wireless interface (ral0), and forward these requests to the DHCP server 10.42.42.42.